The background of the invention is described with particular reference to the field of directional hearing aid, where the present invention is applied, although not exclusively.
Conventionally, hearing aids utilize two microphones spaced apart at a predetermined short distance in order to capture an incoming sound signal. Such devices are often referred to as a directional hearing aid since the subsequent processing of the two audio inputs results in a better directionality perception by the user of the hearing aid. Similar techniques are applied in a number of applications where there is spatial separation between the desired signal and noise sources. Examples include headsets, speech recognition systems and hands-free telephony in automobiles.
In FIG. 1, there is shown a schematic representation of a prior art hearing aid, which is generally denoted by a reference numeral 10. As depicted in FIG. 1, the device includes two microphones 11a and 11b, two amplifiers 12a and 12b, two analog-to-digital (A/D) converters 13a and 13b, a combiner 15, a digital signal processor (DSP) 16, a digital-to-analog (D/A) converter 17, and a loud speaker 18, which are successively connected. In operation, a sound signal coming from a surrounding environment, for example, from a person to whom a user of the device speaks, is captured by the microphone 11a, in which the sound signal is converted to an electrical analog signal. The electrical analog signal is input to the amplifier 12a, where the analog signal is amplified to a higher specific level. Subsequently, the amplified analog signal is converted to a digital representation (a digital signal) of the sound signal in the A/D converter 13a. Similarly, the other signal path, consisting of the microphone 11b, the amplifier 12b, and the A/D converter 13b, performs the same operation as above to produce another digital representation (digital signal) of the sound signal. The two digital signals are then processed in the combiner 15 where the two digital signals are combined into one single signal. The output signal of the combiner 15 may be further processed in the DSP (digital signal processor) 16 where, for example, the signal is filtered or further amplified according to the specific requirements of the application. Alternatively, the combiner 15 can be incorporated into the DSP 16 such that the signal combining can be done in the DSP.
Finally, the amplified and processed digital signal is converted back to an electrical analog signal in the digital-to-analog converter 17 and then converted into sound waves through the loud speaker 18, or applied directly to another systems as an electrical system from the output of the digital-to-analog converter 17.
With the hearing aid noted above, however, use of matched microphones is required in order to perform a satisfactory directionality enhancement through combination and processing of the two audio signals. In this context, the matched microphones mean that they have equal transfer functions and thus equal magnitude and phase responses in a specified frequency range. The concept of matched microphones will be further described in greater detail in conjunction with the description of the preferred embodiments of the present invention.
Currently, the provision of matched microphones has been attempted by using microphone pairs that have been matched by a microphone manufacturer. That is, the microphone manufacturer produces a number of microphones, followed by pairing of the microphones that have similar magnitude and phase response. The manual handling of the microphones affects their properties, and prevents automation of the manufacturing process. Also, additional costs are incurred in the attempt to match the microphones, though they are only matched within a specified tolerance.
Also, U.S. Pat. Nos. 4,142,072 and 5,206,913 disclose microphone matching technologies. However, none of current methods are expected to be satisfactorily successful.
Therefore, there is a need to solve the problems noted above and also a need for an innovative approach to replace the prior art.